Air quality is a major environmental concern. Despite the Clean Air Act of 1977, a widespread and persistent air pollution problem is the emission of compounds that generate lower atmospheric pollution harmful to the human body and compounds that destroy ozone in the Earth's upper atmosphere. It is calculated that over 100 million Americans currently live in cities which have air pollution levels beyond public health standards. Upper atmosphere ozone is able to absorb harmful ultraviolet radiation that would otherwise penetrate the Earth's atmosphere and result in higher incidences of human skin cancer. The cause of this pollution is largely due to the number and diversity of air pollutant sources, including utilities, industries and transportation.
Volatile organic compound (VOC) emissions contribute significantly to air pollution problems in general and in particular, to the destruction of ozone. For instance, VOCs such as chlorofluorocarbons are known to destroy ozone in the Earth's upper atmosphere. Other VOCs are chemicals that can react with sulfur, carbon and hydrogen compounds which can, in the presence of heat and sunlight, form smog or smog-producing compounds. The Clean Air Act of 1990 sets new standards for air quality control resulting in stricter pollution standards for VOC emissions. Thus, industries are faced with the problem of maintaining desired levels of commercial production while at the same time keeping VOC emissions within the new federal regulations.
In various industries it is necessary to remove oils from the surfaces of metals prior to the use of those metals in various ways, for example, in the manufacture of sheet metal for cars, airplanes, appliances, metal components used in high technology industries, food and beverage containers, etc. It is in the treating and coating of such surfaces that significant VOC pollutant problems arise. Oftentimes, production levels in particular industries are limited by the amount of VOCs that can be emitted in a given period of time. This is precisely the case in the metal container industry. Specifically, in the manufacture of metal containers, metal is stamped, formed, drawn and ironed into a desired container configuration, and are finally coated with inks and exterior and interior coatings. During the forming process, residual quantities of oils are deposited on the surface of the metal which must be removed prior to subsequent use of the metal. For instance, such oils must be removed prior to the application of decorative or protective coatings to the metal.
In conventional processes designed to remove undesired oils from the surface of metal, relatively strong acid or caustic solutions are typically used. Primarily, strong acid solutions consisting of hydrofluoric, sulfuric, phosphoric, nitric, and similar acids are used. For example, U.S. Pat. No. Re 32,661 to Binns discloses a method of finishing aluminum using an acidic aqueous solution having a pH less than 2. Similarly, U.S. Pat. No. 3,969,135 by King discloses a composition and process for finishing aluminum using a low temperature aqueous acidic solution with a pH of less than 2.
While the use of acid and caustic solutions to finish metal is effective to remove oil, the use of such solutions presents substantial problems in that they negatively effect the integrity of the metal. For example, the acidic and caustic finishing agents currently in use in container manufacturing operations etch and pit the metal surfaces being finished. During the drawing and ironing procedures for the manufacture of aluminum beverage containers, manganese-iron particles present in the aluminum may create pits and gouges in the metal. Acidic and caustic finishing agents deepen such pits and gouges, causing the surface of the metal to be etched and pitted. The etching and pitting of metal surfaces resulting from the use of acid or caustic finishing agents increases the total surface area of the metal.
The manufacture of metal containers is typically completed by coating the inside and the outside of the containers with inks and protective coatings. The total amount of coating required to achieve sufficient coverage of a metal surface is directly related to the surface area of the metal to be coated. Pits and etches caused by acidic or caustic finishing agents increase the total surface area to be coated, therefore requiring additional amounts of coating to achieve desired coverage.
The compositions of interior and exterior coatings currently used to coat the interior and exterior surfaces of metal containers include VOC-containing coatings that present significant environmental problems in the container manufacturing industry. Both interior and exterior coatings can contain solvents that are released into the atmosphere as VOCs upon curing of such coatings. Thus, the amount of interior or exterior coating required to sufficiently coat a metal surface is directly proportional to the amount of VOCs released into the atmosphere. In view of the environmental concerns highlighted above, a need exists for a new method for finishing metal surfaces able to decrease the surface area of the finished metal, thereby allowing for a reduction in the amount of necessary coatings applied to such surfaces, leading to a consequent reduction in VOC emissions.